1. Field of the Invention
The present invention relates to a bias control circuit for a radio-frequency power amplifier for use in a digital portable telephone system.
2. Description of the Related Art
In a digital cellular telephone system such as portable telephone or automobile telephone, a TDMA method is generally employed to perform alternate transmission and reception by using the same frequency. According to this method, as shown in FIG. 5 for example, a transmission time slot T, a reception time slot R and an idle time slot I are repeated temporally and sequentially in one channel of a fixed frequency.
If 1 block consisting of such successive transmission time slot T, reception time slot R and idle time slot I has a time length of, e.g., 20 msec, it follows that each of the transmission time slot T, reception time slot R and idle time slot I has an equal duration of 20/3 msec.
Transmission from a mobile station to a base station is performed during the transmission time slot T, and reception from the base station is performed during the reception time slot R. The idle time slot I is used for measuring the electric field intensity.
FIG. 6 shows an example of a transmitting circuit in a conventional mobile station. In the transmitting circuit denoted by reference numeral 10, a sound signal is supplied from a transmitter 11 to a digital processor 12 to be thereby converted into a digital signal which is intermittent every transmission time slot T in conformity to a predetermined format, and the digital signal thus obtained is supplied to a modulator 13 to be converted into a transmission signal S13 through orthogonal modulation. Then the transmission signal S13 is supplied to a transmitting/receiving antenna 17 via a signal line consisting of a radio-frequency power amplifier 14, an isolator 15 and a common antenna coupler 16, so that the signal S13 is transmitted to the base station. The frequency band used for such transmission ranges, e.g., from 940 to 960 MHz.
Since the transmission signal S13 is processed through orthogonal modulation and has an amplitude component, it is necessary for the amplifier 14 to execute power amplification of the signal S13 linearly. For this reason, the amplifier 14 needs to perform its operation in the class A or AB.
However, a power transistor constituting the final stage of the power amplifier 14 deals with a considerably great power, so that the temperature in the transistor fluctuates a lot due to the heat generated therefrom. In general, the operating point of the transistor varies depending on the temperature.
A bias control circuit 20 is incorporated in the transmitting circuit of FIG. 6 so that the idle current of the power transistor in the amplifier 14 is controlled to remain at a predetermined value, thereby enabling the amplifier 14 to operate in the class A or AB regardless of the temperature fluctuation.
Circuits shown in FIGS. 7 to 9 may be contrived to serve as the power amplifier 14 and the control circuit 20 which satisfy the conditions mentioned. For the purpose of simplifying the explanation, there drawings merely represent the theoretical connection or constitution.
In the circuit of FIG. 7, there are shown a power transistor Q1 in the final stage, a transistor Q2 for applying a base bias to the transistor Q1, and a temperature compensating diode D1. A voltage including the terminal voltage of the diode D1 is supplied to the transistor Q2 as a base-emitter voltage thereof, so that the collector current of the transistor Q2 is stably maintained at a predetermined value regardless of any temperature fluctuation. Since such collector current is the base current of the transistor Q1, the idle current (collector current) of the transistor Q1 is also stabilized despite the temperature fluctuation.
In the circuit of FIG. 8, a power amplifying transistor Q3 is provided in the final stage. And the voltage obtained from a diode D3 is supplied as a base bias voltage to the transistor Q3. Therefore the idle current of the transistor Q3 is stably maintained at a predetermined value despite any temperature fluctuation.
And in the circuit of FIG. 9, there are included a power amplifying MOS FET Q4 in the final stage, and a thermistor R4. The terminal voltage of the thermistor R4 is supplied to the MOS FET Q4 as a gate bias voltage thereof. Consequently the idle current of the FET Q4 is stably maintained at a predetermined value regardless of any temperature fluctuation.
The circuit of FIG. 7 is effective when the transistor Q1 operates in the class A, but it is impossible to operate the transistor Q1 in the class AB since the collector current thereof is changed in accordance with the level of the transmission signal S13.
Also in the circuit of FIG. 7, the temperature compensating diode D1 needs to be thermally coupled to the transistor Q2. In the circuits of FIGS. 8 and 9, the temperature compensating diode D3 or the thermistor R4 also needs to be thermally coupled to the transistor Q3 or the FET Q4. However, the transistor Q2 to be thermally coupled in the circuit of FIG. 7 is used merely for control, whereas the transistor Q3 (in FIG. 8) the FET Q4 (in FIG. 9) to be thermally coupled is used for power amplification of the transmission signal S13. Therefore, in the circuits of FIGS. 8 and 9, the mounting or arrangement of the circuits and the component elements thereof need to be particularly designed for averting interference and leakage of the signal S13.
In addition to the above, it is necessary in the circuit of FIG. 9 to adjust the temperature characteristic of the thermistor R4 in conformity with that of the FET Q4, hence raising another problem which can complicate the process of manufacture.